Pyridine-2,3-dicarboxylate derivatives are useful intermediates for the preparation of herbicidal 2-(2-imidazolin-2-yl)nicotinic acids, esters and salts such as those described in U.S. Pat. No. 5,334,576 and U.S. Pat. No. 4,798,619. Literature methods for preparing substituted pyridine-2,3-dicarboxylates include degradative techniques which require hazardous oxidative methods such as nitric acid oxidation or base peroxide oxidation of the 2,3-dialkyl or quinolinic precursors. Conventional de novo syntheses of pyridine-2,3-dicarboxylates which employ oxalacetate diesters, or their metal salts, such as those described in U.S. Pat. No. 5,047,542 and JP 01125768A generally give products in low yield and low purity. The use of halogenated oxalacetate diesters to prepare pyridine-2,3-dicarboylate derivatives, although effective, require the formation of unstable .alpha.-halo-.beta.-keto esters such as diethyl chlorooxalacetate, which are known to thermally decompose, releasing HCl gas and creating potentially hazardous and toxic conditions.
Surprisingly, it has now been found that pyridine-2,3-dicarboxyalte derivatives may be effectively and economically prepared using amino alkoxy(or alkylthio)oxalacetate diester compounds, either as starting materials or as in situ intermediates.
Therefore, it is an object of this invention to provide a safe, effective, economic and environmentally compatible process to manufacture pyridine-2,3-dicarboxylate derivatives.
It is another object of this invention to provide a readily available, easily accessible source of starting materials, useful for said process.
It is a feature of the process of the invention that the major side products are alcohols and thiols which may be readily recovered by distillation or extraction.
It is another feature of the process of the invention that the recovered alcohols and thiols may be recycled to produce additional starting material, resulting in minimum waste.
It is an advantage that the compounds of the invention are thermally and chemically stable over a convenient range of conditions and thus require no special handling and present no particular risk to the handlers or the environment.
Further features and objects of the invention will become apparent in the detailed description thereof set forth hereinbelow.